One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (including carbonaceous particulate) and oxides of nitrogen (NOx). Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions. Unfortunately, in practice, it is found that techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. A variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated upon accumulation therein of particulate material.
Examples of such diesel exhaust particulate filters are to be found in European patent application EP 0 010 384; U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.
In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple physical trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. U.S. Pat. No. 4,427,418 discloses the use of ceramic coated wire or ceramic fibres.
In a broader context, the precipitation of charged particulate matter by electrostatic forces also is known. However, in this case, precipitation usually takes place upon larger planar electrodes or metal screens.
GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant. In addition to removing particulates by oxidation, especially electric discharge assisted oxidation, there is disclosed the reduction of NOx gases to nitrogen, by the use of pellets adapted to catalyse the NOx reduction.
U.S. Pat. No. 5,609,736 discloses a method for decomposing volatile organic compounds using a non-thermal plasma reactor containing a bed of ferroelectric pellets coated with an oxidation catalyst. Also, U.S. Pat. Nos. 3,983,021, 5,147,516 and 5,284,556 disclose the catalytic reduction of nitrogen oxides. However, U.S. Pat. No. 3,983,021 is solely concerned with the reduction of NO to N in a silent glow discharge, the temperature of which is kept below a value at which the oxidation of N or NO to higher oxides of nitrogen does not occur.
Although, so-called contact bodies are used in the process of U.S. Pat. No. 3,983,021, and some of those disclosed may have some catalytic properties, catalysis does not appear to be a necessary feature of the process of U.S. Pat. No. 3,983,021. Other surface properties, such as adsorption on large surface area materials, are the basis of the process of U.S. Pat. No. 3,983,021.
U.S. Pat. No. 5,147,516 does refer to the use of catalysts to remove NOx, but the catalytic materials involved are defined very specifically as being sulphur tolerant and deriving their catalytic activity from their form rather than their surface properties. The operating conditions are very tightly defined. There is no specific mention of the type, if any, of electric discharge involved. All that is disclosed is that the NOx removal depends upon electron-molecule interactions, facilitated by the structure of the ‘corona-catalytic’ materials not the inter-molecular interactions involved in the present invention.
U.S. Pat. No. 5,284,556 does disclose the removal of hydrocarbons from internal combustion engine exhaust emissions. However, the process involved is purely one of dissociation in an electrical discharge of the so-called ‘silent’ type, that is to say, a discharge which occurs between two electrodes at least one of which is insulated. The device described is an open discharge chamber, not a packed bed device. Mention is made of the possible deposition of a NOx-reducing catalyst on one of the electrodes.
The specification of application WO 00/71866 describes the use of a dielectric barrier reactor for the processing of exhaust gases from internal combustion engines to remove nitrogenous oxides, particulate including carbonaceous particulate, hydrocarbons including polyaromatic hydrocarbons, carbon monoxide and other regulated or unregulated combustion products. A feature of this reactor is that the flow of the gaseous medium between entering and leaving the reactor has axial, radial and circumferential components. Reference is made to a gas permeable bed of a dielectric medium contained in the space between the electrodes and adapted to have catalytic properties to increase the efficiency of oxidation of particulates and/or reduction of nitrogen oxides.
In accordance with the invention the efficiency of oxidation of particulates and reduction of nitrogen oxides is improved by using a combination of catalyst materials and plasma discharge. Examples of these materials are metavanadates, pyrovanadates and perovskites. Metavanadate materials have the general formula MVO3 where M is a cation for example an alkali metal cation while metal-substituted metavanadates have the general formula M1−xM1xVO3 where M is a cation for example an alkali metal cation and M1 is a cation for example Cu. Examples of metavanadate (MVO3) materials are KVO3, CsVO3, RbVO3 and CuVO3 and metal-substituted alkali-metal vanadates such as K0.7Cu0.3VO3. Pyrovanadate materials have the general formula M4V2O7 and M4−4xM14xV2O7 where M and M1 are cations. Examples of pyrovanadates (M4V2O7) are CS4V2O7, Rb4V2O7 and K4V2O7.
The use of potassium and caesium vanadate and pyrovanadate materials for the low temperature combustion of carbon in simulated diesel emissions, down to as low as 225° C. for Cs4V2O7 has been discussed by G Saracco et al in a paper ‘Development of Catalysts Based on Pyrovanadates for Diesel Soot Combustion’ in Applied Catalysis B: Environmental, volume 21, 233–242, 1999. Here dry soot almost free of any adsorbed hydrocarbons was generated from an ethyne burner.
The use of copper vanadates CuVO3, Cu3(VO4)3 and copper-substituted potassium vanadate K0.7Cu0.3VO3 as carbon combustion catalysts in the absence and presence of metal chloride salts such as potassium chloride in which an amorphous carbon black powder was a simulant for diesel emissions have also been discussed by V Serra et al in a paper ‘Combustion of carbonaceous materials by Cu—K—V based catalysts. II. Reaction mechanism’ in Applied Catalysis B: Environmental, volume 11, 329–346, 1997. Carbon combustion temperatures as low as 382° C. compared with non-catalytic combustion temperatures of 616° C. were observed.
The materials under investigation in the above two papers were heated but not subjected to external influences such as electric fields in particular those associated with non-thermal plasmas and the design of practical reactors incorporating these materials for use in internal combustion engines was not considered.
Perovskites have the general formula ABO3 where A and B are cations and A is the cation with the larger radius. There are many perovskite materials for example LaMnO3, LaAlO3, SrZrO3, BaCeO3 as discussed by T Shimuzu in a paper ‘Partial Oxidation of Hydrocarbons and Oxygenates Compounds on Perovskite Oxides’ in Catal. Rev-Sci. Eng., volume 34, 355–371, 1992 while other examples include LaNiO3, LaFeO3, LaCrO3, LaCoO3, LaMnO3, BaCrO3, BaCoO3, BaFeO3, BaNiO3, PbTiO3. Substitution by other metal cations of the A and or B sites yields perovskites with the general composition A1−xA1xB1−yB1yO3 examples of which are Ba0.8Sr0.2NiO3, Ba0.8Sr0.2CrO3, La0.8Sr0.2NiO3, La0.9K0.1CoO3, La0.9K0.1FeO3 and La0.6Cs0.4CoO3. The latter composition has been described by Yang et al in a paper Simultaneous catalytic removal of NO and carbon particulates over perovskite-type oxides, in Journal of Industrial and Engineering Chemistry, volume 4, 263–269, December 1998. Another perovskite composition is La0.8Sr0.2Mn0.5Cu0.5O3 as described by Duriez et al in the paper ‘Simultaneous NOx reduction and soot elimination from diesel exhausts on perovskite-type oxide in catalysts’ in Catalysts and Automotive Pollution Control III, Elsevier, 1995, edited by A Frennet and J-M Bastin, pages 137–147. In a general sense A and A1 can include La, Sr, K, Na, Li, Cs, Pb and Ba while B and B1 can include Ni, Cr, Co, Mn, Ti, Zr, Ce, Cu and V. Layered perovskite materials have the general formula A2−xA1xB1−yB1yO4 or when A=A1 and B=B1, A2BO4 and examples are described in a paper by Y Tersoka et al in a paper ‘Simultaneous Catalytic Removal of NOx by Diesel Soot and NOx by Perovskite-Related Oxides’, in Catalysis Today, volume 27, 107–115, 1996 and in WO99/38603. Perovskite compositions are also described by VI Parvuleac et al in a paper ‘Catalytic renoval of NO’ in. Catalysis Today, volume 46, 233–316, 1998.
Perovskite materials of the form ABO3 have been evaluated for the simultaneous removal of nitrogenous oxides and diesel soot particulates obtained by the incomplete combustion of diesel fuel as described by Y Teraoka et al in a paper ‘Simultaneous Removal of Nitrogen Oxides and Diesel Soot Particulates Catalysed by Perovskite-Type Oxides’ in Applied Catalysis B: Environmental, volume 5, L181–1185, 1995.
WO 98/32531 discloses use of a perovskite catalyst in combination with plasma in the formation of oxygenates or higher level hydrocarbons from methane
Perovskite materials of the form ABO3 have also been used for the conversion of oxidative and reductive gases, in exhaust gases to harmless gases as described in EP 0089199 A2 (‘Catalysts for Converting Reductive and Oxidative Gases of Exhaust Gases into Innoxious Gases’).
Perovskites were of the form La(1−x)/2Sr(1+x)/2Co1−xMexO3 in which Me is an element selected from the group consisting of Fe, Mn, Cr, V and Ti and x is a number between 0.15 and 0.90. Conversion of CO was carried out in a lean atmosphere containing oxygen while conversion of a mixture of CO, NO2 and gaseous hydrocarbon or a mixture of CO and NO2 was successfully achieved by the perovskite material at for example 300° C. for the conversion of CO alone. EP 0089199 does not describe the use of perovskite materials for conversion of diesel soot particulates in an exhaust gas.
Perovskite materials under investigation in the above examples were heated but not subjected to external influences such as electric fields in particular those associated with non-thermal plasmas and the design of practical reactors incorporating these materials for use in internal combustion engines was not considered by these authors.